1. Field of the Invention
The present invention relates to an optical fiber amplifier. The present invention may be applied in an optical communication system using the wavelength division multiplexing method.
2. Description of Related Art
An optical fiber amplifier is used in, for example, an optical communication system using the wavelength division multiplexing method. In the wavelength division multiplexing method, one wavelength is used as one communication channel, and light of multiple wavelengths is carried simultaneously in a single optical fiber. As a result, an optical fiber amplifier is constructed such that it can amplify light of multiple wavelengths simultaneously.
As such an optical fiber amplifier, an optical fiber amplifier that uses erbium doped optical fiber (EDF) is known in the art. In this amplifier, input signal light and pumping light are input to an EDF. The power of the signal light output from this amplifier is controlled by the power of the pumping light. As optical fiber amplifiers of this type, amplifiers that amplify light in the C-band wavelength region (1530-1565 nm) and those that amplify light in the L-band region (1570-1600 nm) are known.
The EDF used in a L-band amplifier has the same composition as the EDF used in an C-band amplifier, but a different total length. While the length of a C-band EDF is 20-30 meters, the length of an L-band EDF is 100-300 meters. Consequently, even where the total absorption loss for a C-band EDF is large, it is only around 80 dB (decibels), while the total for an L-band EDF comprises 200-600 dB.
In a C-band EDF, the relationship between the power of the pumping light and the power of the output signal light may be expressed as a linear function. In other words, the power of the output signal light depends solely on the power of the pumping light, and does not depend on other factors such as the power of the input signal light. For example, if the power of the pumping light is cut in half, the power of the output signal light is halved as well. On the other hand, with an L-band EDF, the relationship between the power of the pumping light and the power of the output signal light cannot be expressed as a linear function. In other words, the power of the output signal light depends not only on the power of the pumping light, but also on other factors such as the power of the input signal light. This difference in the characteristics of the C-band EDF and the L-band EDF is thought to be due to differences in the pumping mechanism and the length of each EDF. In a C-band EDF, pumping light is directly converted into C-band energy. In an L-band EDF, on the other hand, pumping light is first converted into C-band energy, and is then converted into L-band energy. The efficiency of the conversion of the energy from C-band to L-band energy changes dynamically depending on the power of the input signal light.
In an optical communication system, the output connector of the optical fiber amplifier is connected to optical fiber for communication. When the output connector is connected to the optical fiber, there is no danger of external leakage of the amplified light. However, the communication fiber is sometimes removed by the worker. When the communication fiber is removed even while the optical amplifier is operating, the output connector emits the amplified signal light to the outside. Generally, light amplified by the optical fiber amplifier is extremely powerful, and consequently there is a risk that the amplified light will have a harmful effect on the human body.
In order to protect the human body from the light output from the optical fiber amplifier, it is preferred that the connection/disconnection status of the output connector be automatically detected, the power of the output light be reduced to a level at which it will not be harmful to the human body if the optical fiber is disconnected, and the output power be restored to its normal level when the optical fiber is reconnected. This type of control is not difficult for a C-band amplifier, because in a C-band amplifier, the power of the output signal light has a linear relationship to the power of the pumping light, as described above. In other words, in a C-band amplifier, by simply reducing the power of the pumping light to a pre-set value, the power of the output light can be reduced to a level at which it is not harmful to people. Conversely, it is not easy to perform the above control in an L-band optical amplifier, because in an L-band optical amplifier, the power of the pumping light and the power of the output light do not have a linear relationship, as described above. In a conventional art pertaining to an L-band amplifier, in order to ensure human safety, complex processing using high-order functions is required. To carry out these complex processing, a microprocessor is necessary, which increases the cost of the L-band optical amplifier.
A object of the present invention is to provide an optical fiber amplifier that is very safe for human beings at low cost.
To achieve this object, the optical fiber amplifier pertaining to the present invention comprises: an optical fiber in which the signal light is amplified in response to the power of the pumping light; a front incident means that causes the signal light and the forward pumping light to enter the front end of the optical fiber; a rear incident means that causes the backward pumping light to enter the rear end of the optical fiber; an output means that outputs to the outside the signal light that exits the rear end of the optical fiber; and a control means that, when the output means is in a connected state, sets the power of the forward pumping light to a first prescribed value and sets the power of the backward pumping light to a second prescribed value, and when the output means is in a disconnected state, sets the power of the forward pumping light to a third prescribed value that is smaller than the first prescribed value but larger than zero, and sets the power of the backward pumping light to zero.
By setting the power of the backward pumping light to zero, the power of the signal light output from the output means is made to depend solely on the power of the forward pumping light. As a result, the power of the signal light output from the output means can be reduced to a level safe for human beings using a simple control process.